1. Field of the Invention
The present invention generally relates to a resonance converting apparatus, more particularly to the resonance converting apparatus with function of synchronous rectification, and a synchronous rectification circuit.
2. Description of Related Art
A resonance converting apparatus is applicable to a power-source product. This apparatus may be operated with a synchronous rectification circuit for increasing its efficiency. So that, the driving efficiency of the synchronous rectification circuit may directly affect the power conversion efficiency of the resonance converting apparatus, or even the stability as under a light loading or no loading.
Most of the current resonance converting apparatuses utilize diodes as the synchronous rectification circuit. Reference is made to FIG. 1 showing a schematic diagram of the conventional half-bridge LLC resonance converting apparatus. The shown resonance converting apparatus includes a half-bridge converter 91, a resonant circuit 92, a synchronous rectification circuit 93, and an output circuit 94. The half-bridge converter 91 is constructed of a first switching transistor Q1 and a second switching transistor Q2, and further connected to a midst of a voltage source Vin and a resonant circuit 92. The first switching transistor Q1 and the second switching transistor Q2 operate interactively in accordance with a switching signal (HVG and LVG). Further, this resonant circuit 92 includes a transformer Tr and is formed as an LLC-type resonant circuit. The second side of the transformer Tr further has a first winding and a second winding. The LLC-type circuit is constituted of a resonance inductance Lr, a magnetizing inductance provided by the primary side of the transformer Tr, and a resonance capacitance Cr.
The synchronous rectification circuit 93 further has a first rectification diode SD1 and a second rectification diode SD2, which respectively correspond to the first winding and the second winding, connected to the output circuit 94. Therefore, the interactive operation between the primary side of the first switching transistor Q1 and the second switching transistor Q2 can transfer energy from this primary side to the second side.
However, instead of the rectification diode, the rectification transistor is adopted with a gate-electrode driving IC in the current development since the diode used in the synchronous rectification circuit may cause higher conduction loss. Reference is made to FIG. 2 illustrating the electrical connection between the rectification transistor of conventional synchronous rectification circuit and the driving IC. In discussion about the synchronous rectification circuit 93′, the first rectification transistor SR1 and second rectification transistor SR2 are used to replace the original first rectification diode SD1 and the second rectification diode SD2, and separately connected with one driving IC. After that, the driving IC drives the circuit 93′.
In FIG. 2, only one driving IC represents the structure of the IC and the first rectification transistor SR1 (or second rectification transistor SR2). The first rectification transistor SR1 and second rectification transistor SR2 are formed with a MOSFET (Metallic Oxide Semiconductor Field Effect Transistor). The driving IC is used to examine the current indirectly by examining a drain-source voltage (Vds) of the first rectification transistor SR1 or the second rectification transistor SR2. According to the examined signal, a driving signal (SR1_D or SR2_D) is generated or shutdown for controlling the first rectification transistor SR1 or the second rectification transistor SR2 being turned on or off.
Nevertheless, since the packaged inductance and lead parasitic inductance (Lσ1 and Lσ2) of the winding on the circuit board of the mentioned MOSFET will affect the result examined by the driving IC, the driving signal SR1_D (or SR2_D) generated by the IC may cause shutdown in advance. Therefore, the synchronous rectification of the resonance converting apparatus is inefficient, and also influences the conversion efficiency of the resonance converting apparatus.
Further referring to FIG. 3 that depicts an operating waveform diagram of the conventional half-bridge LLC resonance converting apparatus. FIG. 3 clearly describes the operation while the half-bridge LLC resonance converting apparatus works with the rectification transistor and the driving IC. More, since the first half period and the later half period of the half-bridge LLC resonance converting apparatus are symmetric, the first half period can be the example for the further discussion.
Assuming the elements mounted on the half-bridge LLC resonance converting apparatus are under ideal conditions, the analyses for every condition based on the timing are as follows:
Condition One (t0˜t1):
One resonance period starts at time t0. At time t0, the first switching transistor Q1 and the second switching transistor Q2 are turned off. Meanwhile, a resonance current (iLr) firstly flows through a junction capacitance (not shown) of the first switching transistor Q1 until the drain-source voltage (Vds) of the first switching transistor Q1 becomes zero. (Further, the resonance current (iLr) flows through the diode (not shown) in itself of the first switching transistor Q1. In which the resonance current (iLr) is getting increasing as in a type of sinusoidal wave. The current (iLm) of the magnetizing inductance, in the meantime, increases linearly.
In another aspect, since the voltage signal of the secondary side of the transformer Tr starts to invert, the second rectification transistor SR2 begins turning on. Meanwhile, the voltage of the magnetizing inductance is clamped by an output voltage Vout, and therefore only its resonance inductance Lr and the resonance capacitance Cr produces resonance. More, the resonance current (iLr) is greater than the current (iLm) through the magnetizing inductance, and becomes the current (iSR2) passing through the second rectification transistor SR2. At last under time t1, the switching signal HVG will control the first switching transistor Q1 to be turned on at zero voltage.
Condition Two (t1˜t2):
Under time t1 to t2, the resonance current (iLr) flows through the channel to the first switching transistor Q1 directly and begins increasing the current as well as turning on the first switching transistor Q1. Moreover, operations of the remaining components are the same with the operations under the condition one. However, as in the description above, since the mentioned packaged inductance and the lead parasitic inductance (L σ 1 and L σ 2) of the winding on the circuit board of the MOSFET may affect the current examination result, the driving IC may shut down the driving signal SR2_D in advance of time T. Possibly the diode of the second rectification transistor SR2 will be used to process rectification, and the conduction loss may be higher and cause reduction of the rectifying efficiency.
Until the time t2, the resonance current (iLr) and the current, through magnetizing inductance are the same, and no energy will be transferred on the transformer Tr. Further, the current (iSR2) through the second rectification transistor SR2 reaches zero.
Condition Three (t2˜t3):
Generally the first switching transistor Q1 is turned on constantly. Since the resonance current (iLr) is greater than zero and equal to the current (iLm) through the magnetizing inductance at this moment, there is not any energy being transferred as the transformer Tr is regarded as open circuit. Meanwhile, the voltage of magnetizing inductance won't be clamped by the output voltage Vout. Therefore, the magnetizing inductance will join the resonance caused by the resonance inductance Lr and the resonance capacitance Cr. Since the condition three ends, the first switching transistor Q1 is controlled to be turned off under zero voltage
From above description, although a rectification transistor is incorporated into the current resonance converting apparatus for processing synchronous rectification, the synchronous rectification driver may not obtain better efficiency since the driving IC uses the drain-source voltage of the rectification transistor to indirectly examine the current. The efficiency needs more improvement.